1. Field of the Invention
The present invention relates to an automatic document feeder and a scanning apparatus therewith, and more particularly, relates to an automatic document feeder and a scanning apparatus able to detect a moving speed of a document and to control the scan timing accordingly.
2. Description of the Prior Art
Nowadays, scanners on the market can be equipped with an automatic document feeder for transporting a document or a stack of sheet past a stationary image-reading assembly automatically, so as to scan documents of multiple pages conveniently.
Please refer to FIG. 1. FIG. 1 illustrates an automatic sheet-fed scanning apparatus 1 of the prior art. The scanning apparatus 1 includes a feeding passage 10, a feeding roller 12 and transporting roller assemblies 141, 142, 143, and 144. The feeding roller 12 is driven by a motor for feeding a document 2 into the feeding passage 10. Afterwards, the transporting roller assemblies 141, 142, and 143, driven by the motor, transport the document 2 through the feeding passage 10. When the document 2 is transported past an image-reading assembly 16, the image-reading assembly 16 reads the context of the document 2.
The image-reading assembly 16 includes optical devices (for example, a light source and lenses) and a sensing device that may comprise charge-coupled device (CCD) sensors or contact image sensors (CIS). The moving speed of the document 2 past the scanning area 160 affects the quality of a scan image. Thus, if a high quality scan image is desired, the moving speed of the document 2 transported in the feeding passage 10 has to be constant while the document 2 passes by the scanning area 160.
Although the transporting roller assemblies 141, 142, 143, and 144 are driven by the same motor, the actual moving speed of the document 2 in the feeding passage 10 differs from a predetermined speed and the moving speeds of the document 2 at different locations of the feeding passage 10 are also different due to the differences of the diameters of the transporting roller assemblies 141, 142, 143, and 144 and the pressures of springs on the roller assemblies 141, 142, 143, and 144. Particularly, when the document 2 is transported past the scanning area 160, because the rotational speeds of the transporting roller assembly 143 and the transporting roller assembly 144 are different, which will lead to the fact that the widths of the scan lines of the image generated are different, the scan image will be distorted. For example, if the vertical scanning resolution is 600 dpi, the width of one scan line should be 1/600 inch. However, when the document 2 is transported past the scanning area 160 at a speed higher or slower than a predetermined speed, the width of the scan line is shorter or longer than 1/600 inch.